Improve prompt message on UserManager for force-sign-in user

Bug: 
Change-Id: Ib1f3a413560ba926cfb0bf1a4704b84f8ec69847
Reviewed-on: https://chromium-review.googlesource.com/780184
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Georges Khalil <georgesak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518440}

chrome/app/generated_resources.grd

@@ -9713,7 +9713,7 @@ For more information, visit our <ph name="BEGIN_LINK">&lt;a target="_blank" href
             Please unlock your profile before adding a person.
           </message>
           <message name="IDS_USER_MANAGER_PROMPT_MESSAGE" desc="Prompt message displayed on UserManager when force-sign-in policy is enabled.">
-            Your administrator requires you to sign in to Chrome.
+            To use Chrome, please sign in.
           </message>
           <message name="IDS_LOGIN_POD_USER_REMOVE_WARNING_NONSYNC" desc="Main text shown as a warning when attempting to remove an user.">
             This will permanently delete your browsing data from this device.